This invention relates to a method and apparatus to measure relative density of a gas.
The gas may be fuel gas, for example natural gas.
According to a first aspect of the invention a method of measuring the relative density of a gas comprises making a measure of the speed of sound in the gas and using the speed of sound in an operation producing the relative density of the gas corresponding to said speed of sound.
Said method may comprise making a measure of the speed of sound in the gas corrected to standard conditions and obtaining the relative density by a procedure involving use of the formula
RD=axc3x97SoSsc+b,
where RD is the relative density of the gas, SoSsc is the speed of sound in the gas corrected to standard conditions (sc), and a and b are constants.
The speed of sound in the gas may be corrected to metric standard conditions (msc) whereby the formula may be written as
RD=axc3x97SoSsc+b,
According to a second aspect of the invention apparatus to measure the relative density of a gas comprises means to measure the speed of sound in the gas, and means to use the speed of sound in an operation producing the relative density of the gas corresponding to said speed of sound.
Said apparatus may comprise means to measure the speed of sound SoSsc in the gas corrected to standard conditions, and means to obtain the relative density by a procedure involving use of the formula
RD=axc3x97SoSsc+b,
In said apparatus the speed of sound may be corrected to standard metric conditions.
The method and apparatus may be adapted to use a measure of the calorific value of the gas to calculate the Wobbe Index of the gas using the formula       WI    =          CV              RD              ,
where WI is the Wobbe Index and CV is the calorific value of the gas.